I was trying to decide whether to go with the 2.26GHz or 2.53GHz model too, and I finally decided on the 2.26GHz.
Why I picked the 2.26? To save up extra cash until late 2009 to upgrade to an SSD and Snow Leopard.
An SSD and Snow Leopard (and possibly 4GB of RAM if needed) will be far more rewarding than a minor clock boost. The $300 spent on the 2.53GHz model could be doubled to pay for an SSD drive in the future (assuming they become cheaper).
The 2.26GHz processor can do everything the 2.53GHz processor can do, anyways. The computer is already a fantastic piece of machinery. 64-bit processor, dual core, what more could you ask for? With the power-boost from Snow Leopard and the SSD, it'll be even better.